An unexpectedly wide variety of oncogenes have been found to contribute to the induction of murine B lineage lymphomas. Bone marrow or fetal liver cells transformed in vitro with retroviruses containing K-ras, H-ras, bas, fes, src, and erbB were all shown to yield cell lines with characteristics of pre-B cell lymphomas. Detailed studies of some of these tumors and others indicated a close relationship between the B cell and myelomonocytic pathways of differentiation. Cells with many characteristics of mature macrophages were shown to be clonally related to cells with pre-B characteristics. Studies of tumors classified previously as plasmacytoid lymphosarcomas showed that they are myelomonocytic leukemias representing different stages of differentiation. Treatment of the cells with PMA resulted in maturation in the face of high levels of myb expressive. Myb expression was shown to be driven from the inserted retroviral LTRs. Newborn NFS mice inoculated with retroviruses containing an avain v-myc gene developed T cell, B cell lymphomas and mammary and pancreatic adenocarcinomas. The same viruses inoculated into adult pristane-primed BALB/c mice produced plasmacytomas ans myelogenous leukemias. By comparison, treatment of pristane-primed mice with retroviruses containing a murine c-myc gene resulted exclusively in the development of myeloid tumors.